The present invention is generally concerned with the preparation of a feedstock for carbon artifact manufacture from carbonaceous residues of petroleum origin including distilled or cracked residium of crude oil and hydrodesulfurized residues of distilled or cracked crude oil and to the use of that feedstock for carbon artifact manufacture, including fiber preparation.
Carbon artifacts have been made by pyrolyzing a wide variety of organic materials. It should be appreciated that this invention has applicability to carbon artifact formation generally and most particularly to the production of shaped carbon articles in the form of filaments, yarns, films, ribbons, sheets and the like.
The use of carbon fibers in reinforcing plastic and metal matrices has gained considerable commercial acceptance where the exceptional properties of the reinforcing composite materials such as their higher strength to weight ratio clearly offset the generally high costs associated with preparing them. It is generaly accepted that large-scale use of carbon fibers as a reinforcing material would gain even greater acceptance in the marketplace, if the costs associated with the formation of the fibers could be substantially reduced. Thus, the formation of carbon fibers from relatively inexpensive carbonaceous pitches has received considerable attention in recent years.
Many carbonaceous pitches are known to be converted at the early stages of carbonization to a structurally ordered optically anisotropic spherical liquid called mesophase. The presence of this ordered structure prior to carbonization is considered to be significant in determining the fundamental properties of any carbon artifact made from such a carbonaceous pitch. Indeed, the ability to generate high optical anisotropicity during the early processing steps is accepted particularly in carbon fiber production as a prerequisite to the formation of high quality products. Therefore, one of the first requirements of any feedstock material suitable for carbon artifact manufacture and particularly carbon fiber production is its ability to be converted to a highly optically anisotropic material.
In addition to being able to develop a highly ordered structure, suitable feedstocks for carbon artifact manufacture and particularly carbon fiber manufacture should have relatively low softening points, rendering them suitable to being deformed, shaped or spun into desirable articles. For carbon fiber manufacture, a suitable pitch which is capable of generating the requisite highly ordered structure must also exhibit sufficient viscosity for spinning. Unfortunately, many carbonaceous pitches have relatively high softening points. Indeed, incipient coking frequently occurs in such materials at temperatures where they have sufficient viscosity for spinning. The presence of coke or other infusable materials and/or undesirably high softening point components generated prior to or at the spinning temperatures are detrimental to processability and are believed to be detrimental to product quality. For example, U.S. Pat. No. 3,919,376 discloses the difficulty in deforming pitches which undergo coking and/or polymerization near their softening temperatures.
Another important characteristic of a feedstock for carbon artifact manufacture is its rate of conversion to a suitable optically anisotropic material. For example, in the above-mentioned U.S. patent, it is disclosed that 350.degree. C. is the minimum temperature generally required to produce mesophase from a carbonaceous pitch. More importantly, however, is the fact that at least one week of heating is necessary to produce a mesophase content of about 40% at that minimum temperature. Mesophase, of course, can be generated in shorter times by heating at higher temperatures. However, as indicated above, at temperatures particularly in excess of about 425.degree. C., incipient coking and other undesirable side reactions do take place which can be detrimental to the ultimate product quality.
It has become known that typical graphitizable carbonaceous pitches contain a separable fraction which possesses very important physical and chemical properties insofar as carbon fiber processing is concerned. Indeed, the separable fraction of typical graphitizable carbonaceous pitches exhibits a softening range or viscosity suitable for spinning and has the ability to be converted at temperatures in the range generally of about 230.degree. C. to about 400.degree. C. to an optically anisotropic deformable pitch. Unfortunately, the amount of separable fraction present in well known commercially available graphitizable pitches such as Ashland 240 and Ashland 260, to mention a few, is exceedingly low. For example, with Ashland 240, no more than about 10% of the pitch constitutes a separable fraction capable of being thermally converted to a liquid crystalline phase.
It has also become known that the amount of the fraction of typical graphitizable carbonaceous pitches which exhibits a softening point and viscosity suitable for spinning and has the ability to be rapidly converted to low temperatues to highly optically anisotropic deformable pitch can be increased by heat soaking the pitch, for example at temperatures in the range of 350.degree. C. to 450.degree. C., until spherules visible under polarized light begin to appear in the pitch. The heat soaking of such pitches has generally resutled in an increase in the amount of the fraction of the pitch capable of being converted to an optically anisotropic phase. Indeed, yields up to about 48% of a separable phase were obtained upon heat treatment of the Ashland 240, for example.
It is disclosed in U.S. Pat. No. 4,219,404 that polycondensed aromatic oils present in isotropic carbonaceous feedstocks are generally detrimental to the rate of formation of highly optical anisotropic material in such feedstocks when heated at elevated temperatures and such polycondensed aromatic oils can be readily removed by techniques such as vacuum or steam stripping or the like. Heat soaking such pitches in which at least a portion of the amount of aromatic oils have been removed results in high yields of a feedstock suitable for carbon artifact manufacture. The patent further discloses that such a pitch can thereafter be treated with a solvent or mixture of solvents which will result in the separation of the solvent insoluble fraction of the pitch which is highly anisotropic or capable of being converted to a highly anisotropic phase and which has a softening point and viscosity at temperatures in the range of about 250.degree. C. to about 400.degree. C. which is suitable for spinning.
Copending Application Ser. No. 475,068, filed Mar. 14, 1983 (PCO-33-82), similarly assigned, teaches that there is a particular fraction of the distillable oil removed carbonaceous residue of petroleum origin of the aforementioned patent, U.S. Pat. No. 4,219,404, which can be recovered by suitable means and converted into a precursor feed stock material that exhibits a softening point and viscosity which is suitable for spinning and has the ability to be rapidly converted at low temperatures to highly optical anisotropic deformable pitch. That fraction exhibits a reversed solubility curve and is obtained by subjecting the heat-soaked, distillation oil removed carbonaceous residue to a two-stage extraction in an organic solvent to take advantage of the reverse solubility curve followed by heat-soaking at atmospheric pressure.
It has now been discovered that the distillation oil removed carbonaceous residue of petroleum origin whcih has been solvent extracted as described in the aforementioned U.S. Pat. No. 4,219,404, or in my copending application can be further improved by an additional heat treatment step at reduced pressure to provide a precursor feed stock material that exhibits a softening point and viscosity which is suitable for spinning and has the ability to be rapidly converted at low temperatures to highly optical anisotropic deformable pitch.
It is, accordingly, the object of this invention to provide a method of obtaining a pitch having a softening point and viscosity suitable for spinning and to provide spun products from such a pitch. This and other objects of the invention will become apparent to thsoe skilled in the art from the following detailed description of the invention.